Production of alpha-quartz-cristobalite silica for pottery

ABSTRACT

There is described a process for the production of silica for use as pottery body from silica sand. A mixture of the silica sand and from 0.3 to 2 percent by weight of the mixture of a calcium compound, calculated as calcium oxide, is calcinated at a temperature of from 1,200* to 1,550* C. for 5 to 60 minutes.

United States Patent [1 1 3,660,123 Smith [451 May 2, 1972 541PRODUCTION OF ALPHA-QUARTZ- 2,726,964 12/1955 Smoke..... ..106/46CRISTOBALITE SILICA FOR POTTERY 2,937,102 5/ 1960 Wagner ..106/69 X [72]Inventor: Albert Richard Smith, Redhill, England OTHER PUBLICATIONS [73]Assignee: British Industrial Sand Limited, Surrey, Ceramic Raw Material,in Ceramic Industry Magazine, (Sil- England ica) Chicago, 111.;Jan. 1967pp. 138'-139. [22] Filed Feb 4 1970 Kingery, W. D.; Introduction toCeramics; New York, 1960 p.

' 137. [2]] Appl. No.: 8,613 Searle, A. B.; Refractory Materials;London, 1950 p. 160.

Sosman, R. B.; The Properties of Silica; New York, 1927 p. 78 [30]Foreign Application Priority Data Primary Examiner-Tobias E. Levow Feb.5, 1969 Great Britain ..6,24l/69 Assistant r Satterfield Attorney-Dowell& Dowell [52] U.S. Cl. ..106/39 R, 23/182, 106/69,

264/56 [57] ABSTRACT E2 :I fl' a 5' ggg'ggfgfigl There is described aprocess for the production of silica for 264/56 use as pottery body fromsilica sand. A mixture of the silica sand and from 0.3 to 2 percent byweight of the mixture of a calcium compound, calculated as calciumoxide, is calcinated [56] Reierences cued at a temperature of from1,200" to 1,550 C. for 5 to 60 UNITED STATES PATENTS minutes 3,45 8,3317/1969 Krayer 106/45 7 Claims, 1 Drawing Figure I PATENTEU A 2 I972 RBEqm aw wwwzw z PRODUCTION OF ALPHA-QUARTZ-CRISTOBALITE SILICA FOR POTTERYThis invention relates to the production of silica for pottery and isespecially concerned with a process for preparing a silica suitable forpottery body from silica sands.

BACKGROUND TO THE INVENTION Silica is one of the principle constituentsof. pottery body and the main source of the silica which is incorporatedinthe pottery has been the flint pebbles which occur as, for example,beach deposits in coastal areas and also in limestone and chalk beds ininland areas.

The flint pebbles are calcined so as to bring about a partial conversionto cristobalite of the micro-crystalline alphaquartz which is the mainconstituent of flint. The calcination also burns out any organic matterpresent in the flint and the final product is white. The resultingpebbles are then fragmented and crushed and milled to a fine powderwhich is incorporated in the slurry or slip constituting the potterybody.

The known deposits of flint are becoming progressively exhausted and itwould therefore be advantageous to be able to replace the flint by usingsilica from ordinary silica sand which occurs widely in nature. Suchsilica sand consists of crystalline alpha-quartz and although attemptshave been made in the past to use ground silica sand as a pottery bodythese attempts have been unsuccessful because of the difference inthermal expansion between quartz from sand and calcined flint. This hasresulted in the cracking of thepottery body during firing and even whenthis could be overcome by varying the composition of the pottery body itwas then difficult to suitable glaze for this pottery body.

When quartz is heated to a high temperature in the presence of certainalkali and alkaline earth metal compounds conversion of quartz tocristobalite takes place. The resulting converted material, however,cannot normally be incorporated in a pottery body in place of at leastsome of the calcined flint without encountering difficulties of thermalexpansion,

We have now found that by heating silica sand under particularconditions we can obtain a product which can be satisfactorilyintroduced as a component of pottery body.

prepare a THE INVENTION "calcined at a temperature of from 1,200 tol,550 C. for a time of from 5 to 60 minutes, the proportion of calciumcompound calculated as calcium oxide being from 0.3 to 2 percent byweight of the mixture.

According to the process of the invention a partial conversion of thequartz to cristobalite occurs. The resulting material when ground to thesame fineness as the calcined flint normally used is now found to beperfectly satisfactory for incorporation into pottery body. Also thefinely ground resulting material can be used to prepare a slip which canbe handled in the normal way. This does not happen when silica sand isheated with a sodium salt since then the resulting particles when groundand mixed with water gives a slurry or slip which rapidly phaseseparates and can therefore only be used with great difliculty in thepreparation of pottery articles.

The product of the process is a mixture of alpha-quartz and cristobaliteand its specific gravity is generally from about 2.45 to 2.60 which isof course between the value for alphaquartz which is 2.65 and the valuefor cristobalite which is 2.32. The product of the invention thereforehas a content of cristobalite of from about 60 'to 5 percentrespectively.

The exact content of cristobalite can be varied in a number of ways.With a fixed calcination time and temperature an increased amount ofcalcium compound will increase the degree of conversion of quartz tocristobalite. On the other hand for a fixed addition of calcium compoundmore cristobalite is formed as the temperature or time are increased.

It is found that if more than 2 percent of the calcium compoundcalculated as lime is added then the properties of the slip preparedfrom the resulting finely ground material are unfavourably affected. Inpractice it is usually preferred to use less than 2 percent of thecalcium compound calculated as lime and the preferred range is from 0.3to 1.5 percent.

The preferred calcium compound to be mixed with the silica is calciumoxide or lime since this is readily available and relatively cheap.Other calcium compounds such as the hydroxide or carbonate can also beused however.

The starting silica sand should have a high content of silica, forexample, at least around 98 percent and it can be cleaned of impuritiesbefore use. For example it may be washed to remove clay and otherimpurities and possibly bleached to remove stains and the like.

DESCRIPTION .OF A PARTICULAR EMBODIMENT In one particular embodiment ofthe invention a high silica sand was treated with an aqueous slurrycontaining from 3 to 5 percent by weight of calcium hydroxide. Theexcess liquid was removed by filtration so as to leave 8 to 10 percentmoisture in the sand and 0.3 to 0.5 percent by weight of calcium oxideas the hydroxide mixed with the sand.

This mixture was then calcined by passing it through a rotary kiln, forexample of the type used in the manufacture of cement, where thetemperature was raised to around 1,470 to l,530 C. The size of the kilnwas such that the sand and calcium hydroxide mixture remained in it for20 to 30 minutes.

After discharge and cooling the sand was ground to a suitable finenessand was found to have a specific gravity of'2.56 corresponding to about25 to 30 percent of cristobalite. The resulting ground silica flour wasthen eminently suitable for use in pottery manufacture in thepreparation of a slurry or slip containing the treated silica either asthe main component or in place of part of the calcinedbflint withoutproblems of differing thermal expansion and resulting cracking. A

The attached drawing is a flow diagram illustrating the above describedembodiment of the invention.

An additional advantage of the material produced according to theinvention is that it is very white. Whereasymost silica sands have ayellow or pink colour the reaction with the calcium compound duringcalcination appears to remove almost all traces'of colour and leaves aproduct which is very white in appearance. This is of course animportant advantage in the manufacture of pottery since it reduces orobviates the necessity of using opaque glazes to make the pottery bodycolourless.

The calcium compound can of course be mixed with thesilica sand in anysuitable way. We have described above how the sand can be mixed with aslurry of calcium hydroxide. Alternatively the dry calcium salt anddried sand can be mixed. When a slurry is prepared, excess liquid can beremoved in any suitable way such as by natural draining or centrifuging.

Also the type of furnace in which the mixture is calcined is notcritical provided of course the desired temperature range is achievedfor the specified time period. Besides rotary kilns, tunnel kilns,fluidised bed furnaces or sinter strands can be used. Also the way inwhich heat is supplied to the mixture is immaterial and can for examplebe by the combustion of fuel oil, town or producer gas or natural gas orby indirect heating such as electrical heating.

A latitude of modification, change and substitution is intended in theforegoing disclosure and in some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly it is appropriate that the appended claims beconstrued broadly and in a manner consistent with the spirit and scopeof the invention herein.

I claim:

1. A process for the production of silica in the form of a mixture ofAlpha quartz and cristobalite having a specific gravity of from about2.45 to 2.60 which comprises the step of calcining a mixture of silicasand and a calcium compound at a pound is one chosen from the groupconsisting of calcium hydroxide and calcium carbonate.

5. A process according to claim 1 in which said silica sand has acontent of silica of at least 98 percent.

6. A process according to claim 1 in which said mixture is calcined at atemperature of around 1,470 to 1,530 C.

7. A process according to claim 6 in which said mixture is calcined fromfrom 20 to 30 minutes.

2. A process according to claim 1 in which the proportion of saidcalcium compound calculated as calcium oxide is from 0.3 to 1.5 percentby weight of said mixture.
 3. A process according to claim 1 in whichthe calcium compound is calcium oxide.
 4. A process according to claim 1in which the calcium compound is one chosen from the group consisting ofcalcium hydroxide and calcium carbonate.
 5. A process according to claim1 in which said silica sand has a content of silica of at least 98percent.
 6. A process according to claim 1 in which said mixture iscalcined at a temperature of around 1,470* to 1,530* C.
 7. A processaccording to claim 6 in which said mixture is calcined from from 20 to30 minutes.